The present invention relates to a process for extracting metal values from manganiferous ocean floor nodule ore by means of hydrochloric acid.
Manganiferous ocean floor nodule ore comprises primarily oxides of tetravalent manganese and trivalent iron and secondarily other metal compounds, generally oxides, including those of copper, nickel and cobalt and, often, other elements such as magnesium, aluminum, zinc, vanadium and molybdenum.
According to known processes, crushed ocean floor nodule ore is contacted countercurrently in a five-stage reactor with an aqueous HCl solution having an initial concentration of 11 N. The chlorine resulting from the MnO.sub.2 -HCl reaction is vented from each stage. Additional HCl gas may be added to one or more stages. From the aqueous leach solution leaving the final stage, Fe, Cu, Ni and Co are selectively extracted. The remaining manganiferous solution is then electrolyzed or treated for obtaining anhydrous manganese chloride, which is transformed into manganese metal in an aluminum reduction cell. The extracted FeCl.sub.3 can be used to prechlorinate the ore or is reacted with water vapor to form iron oxide and HCl which is recycled to the five-stage reactor. By-product aluminum chloride from the reduction cell is also reacted with water vapor to form aluminum oxide and HCl which is recycled to the reactor.
These prior art processes present several drawbacks. They require a complicated leaching apparatus and a highly concentrated leaching reagent, which is always expensive, even if produced from recycled HCl. If recycled HCl, which is normally diluted with combustion gases, is directly added to the leaching reactor, the chlorine vented from the reactor will be diluted with the same gases. No clear-cut separation is provided between manganese on the one hand and the alkaline and alkaline-earth metals on the other, so that considerable amounts of manganese and chloride ions are lost either in rejected mother liquors or in drainings. No regeneration is provided for the neutralizing agent which is used in some of the selective extraction stages. Moreover, no recycling is provided for chlorine vented from the leaching reactor.
A well-known method for demanganizing a solution, e.g., a chloride solution, consists in treating the solution with chlorine in order to oxidize Mn++ into insoluble MnO.sub.2, while adding a base, e.g., MgO, to the solution in order to neutralize the generated HCl according to the equations: ##EQU1##
The above method has been proposed in U.S. Pat. No. 3,770,868 as part of a cyclic process for preparing synthetic manganese dioxide from high-grade terrestial manganese dioxide ore, in which chlorine generated during the hydrochloric acid leaching of the ore is used together with make-up chlorine to precipitate manganese dioxide in the purified leach solution, the pH of which is maintained above 0.5, preferably between 0.5 and 3, and in which magnesia and hydrochloric acid or magnesium hydroxide and hydrochloric acid are regenerated by spray roasting of the final magnesium chloride solution.
Applying such a cyclic process to ocean floor nodule ore treatment, the primary aim of which is to produce high-value non-ferrous metals such as copper, nickel and cobalt, would lead to the production of large quantities of manganese dioxide, for which there is only a small market; therefore, all or the major part thereof would have to be reduced directly or indirectly into manganese metal. Moreover, since up to 15% of the manganese contained in the nodule ore may be in the bivalent state and does not generate chlorine in the leaching step, excessive amounts of make-up chlorine would be required in the demanganizing step.